Hilo

Tonight: Mostly cloudy. Numerous showers in the evening, then occasional showers after midnight. Lows around 68 near the shore to around 54 at 4000 feet. North winds up to 10 mph shifting to the west after midnight. Chance of rain 90 percent.

Monday: Occasional showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs 79 to 85 near the shore to 67 to 72 at 4000 feet. Southeast winds up to 15 mph. Chance of rain near 100 percent.

Monday Night: Occasional showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows around 71 near the shore to around 57 at 4000 feet. Southeast winds up to 10 mph. Chance of rain near 100 percent.

Kona

Tonight: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows 68 to 74 near the shore to around 54 near 5000 feet. Light winds. Chance of rain 40 percent.

Monday: Partly sunny. Isolated showers and slight chance of thunderstorms in the morning, then scattered showers and slight chance of thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs around 85 near the shore to around 72 near 5000 feet. Light winds. Chance of rain 50 percent.

Monday Night: Mostly cloudy with slight chance of thunderstorms and isolated showers. Lows 71 to 77 near the shore to around 57 near 5000 feet. Light winds. Chance of rain 20 percent.

Waimea

Tonight: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows around 67 near the shore to 54 to 61 near 3000 feet. East winds up to 15 mph. Chance of rain 70 percent.

Monday: Breezy. Occasional showers in the morning, then occasional showers and slight chance of thunderstorms in the afternoon. Locally heavy rainfall possible. Highs around 81 near the shore to 72 to 79 near 3000 feet. Northeast winds up to 20 mph. Chance of rain near 100 percent.

Monday Night: Occasional showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows around 70 near the shore to 57 to 64 near 3000 feet. Southeast winds up to 15 mph. Chance of rain near 100 percent.

Kohala

Tonight: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows around 67 near the shore to 54 to 61 near 3000 feet. East winds up to 15 mph. Chance of rain 70 percent.

Monday: Breezy. Occasional showers in the morning, then occasional showers and slight chance of thunderstorms in the afternoon. Locally heavy rainfall possible. Highs around 81 near the shore to 72 to 79 near 3000 feet. Northeast winds up to 20 mph. Chance of rain near 100 percent.

Monday Night: Occasional showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows around 70 near the shore to 57 to 64 near 3000 feet. Southeast winds up to 15 mph. Chance of rain near 100 percent.

South Big Island

Tonight: Breezy. Partly cloudy with isolated showers. Lows around 68 near the shore to around 53 near 5000 feet. Northeast winds up to 25 mph. Chance of rain 20 percent.

Monday: Mostly cloudy. Windy. Scattered showers and slight chance of thunderstorms in the morning, then numerous showers and slight chance of thunderstorms in the afternoon. Locally heavy rainfall possible. Highs around 84 near the shore to around 73 near 5000 feet. East winds 10 to 30 mph. Chance of rain 70 percent.

Monday Night: Breezy. Cloudy with numerous showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows around 71 near the shore to around 56 near 5000 feet. East winds 10 to 25 mph. Chance of rain 70 percent.

Puna

Tonight: Mostly cloudy. Numerous showers in the evening, then occasional showers after midnight. Lows around 68 near the shore to around 54 at 4000 feet. North winds up to 10 mph shifting to the west after midnight. Chance of rain 90 percent.

Monday: Occasional showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs 79 to 85 near the shore to 67 to 72 at 4000 feet. Southeast winds up to 15 mph. Chance of rain near 100 percent.

Monday Night: Occasional showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows around 71 near the shore to around 57 at 4000 feet. Southeast winds up to 10 mph. Chance of rain near 100 percent.

Waikoloa

Tonight: Mostly cloudy with isolated showers. Lows 69 to 74 near the shore to 50 to 57 above 4000 feet. East winds up to 15 mph. Chance of rain 20 percent.

Monday: Partly sunny. Isolated showers in the morning, then scattered showers and slight chance of thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs around 84 near the shore to 68 to 74 above 4000 feet. Northeast winds up to 10 mph shifting to the northwest in the afternoon. Gusts up to 30 mph. Chance of rain 50 percent.

Monday Night: Mostly cloudy with slight chance of thunderstorms and isolated showers. Lows 72 to 77 near the shore to 53 to 60 above 4000 feet. Light winds. Chance of rain 20 percent.

Detailed Forecast

Synopsis

Moderate trade winds will continue through this evening, with clouds and passing showers affecting mainly windward and mauka areas. A plume of tropical moisture will bring the potential for locally heavy rain and a few thunderstorms tonight through Tuesday, particularly over Maui and the Big Island. A cold front then appears to stall over or just north of the islands around the middle of the week while a new high builds to its north. This is expected to bring a period of wet trades and breezy to locally windy conditions to the islands for the latter half of the week.

Discussion

A surface ridge currently centered about 1500 miles northeast of Honolulu is helping to produce moderate trades across the state this afternoon and ushering isolated showers into windward areas. Besides across leeward Big Island and portions of leeward Maui, sea breezes and their associated inland cloud cover and shower development have not materialized as of yet this afternoon.
The main forecast concern in the short-term is the potential for heavy rain and flooding, particularly for windward Big Island and Maui, starting late tonight and lasting through Tuesday. A surface trough about 450 miles southeast of Hilo this afternoon can be clearly seen on visible satellite and will continue to advance towards the eastern end of the state over the next several hours. CIMSS MIMIC satellite imagery shows a broad area of 2+ inches of precipitable water around this trough. As this trough moves by to the south and its plume of moisture moves west- northwestward across the island chain tonight through Tuesday, a weak shortwave trough aloft will also set up over the state and bring increased atmospheric instability. The low level trough, deep tropical moisture, and shortwave trough aloft look to all line up and bring the best chance of heavy rain even a slight chance of thunderstorms to windward Big Island and Maui Monday through Tuesday. As such, a Flood Watch is in effect for Maui and the Big Island from late tonight through Tuesday. However, forecast confidence in the potential for flooding remains only moderate due to model discrepancies. Otherwise, the trades should strengthen a bit tonight and Monday as the surface trough passes south of the state, then weaken again on Tuesday with the approach of the next cold front…which is the next main focus of the forecast.
Model guidance suggests that a cold front will approach and potentially pass through the state sometime Wednesday through Thursday. A new high pressure ridge building in north of the front Tuesday night is expected to lead to a resurgence in the trades, with breezy to locally windy conditions developing by late Wednesday and holding into next weekend. Now for the model discrepancies… The ECMWF is the most aggressive with this front and brings it all the way through the Big Island by Thursday morning. In contrast, the GFS brings it to Kauai's doorstep on Thursday morning and then has it dissipate. With these two significantly different solutions, forecast confidence decreases Wednesday and beyond. For now, will continue to lean towards the GFS's more conservative solution of no major frontal passage and wet trades through the rest of the week into next weekend, but will likely need to make some adjustments as the details become clearer.

Aviation

Moderate trade winds are prevailing this afternoon, but trades should strengthen slightly tonight and Monday as a surface trough passes south of the state. The main aviation concern during the forecast period will be chances for showers, low clouds, and isolated thunderstorms across the eastern end of the state beginning late tonight through Monday. As the surface trough approaches the island chain, an influx of tropical moisture will move over the eastern islands. This could result in widespread MVFR ceilings and visibilities across windward sections of the Big Island and Maui beginning late tonight. Elsewhere, more typical trade wind showers are expected throughout this period.
There are currently no AIRMETs in effect. However, mountain obscuration could become an issue late tonight or early Monday as the trough approaches closer to the Big Island.

Marine

Moderate to fresh trades will briefly strengthen on Monday as a surface trough approaches the state from the east potentially warranting a short-lived Small Craft Advisory for the windier waters of Maui County and the Big Island. This trough will also bring the threat of thunderstorms to the waters around the Big Island and Maui Monday night through Tuesday. Strong trades will then develop Wednesday into Wednesday night. Confidence is lower than normal regarding the potential for this episode of strong trades to persist into the weekend.
Existing long period moderate NW (320) swell is peaking this afternoon and will gradually subside beginning tonight. The High Surf Advisory (HSA) for N and W facing shores looks to be in good shape and will remain in effect through tonight. A series of moderate NW to NNW swells are possible during the second half of the week into early next week.
Surf along east-facing shores will remain small and choppy with some wrap possible from the NW swells at select exposures through the near term. As trades strengthen during the second half of the week, surf along E facing shores will likewise tick upward. Surf along south-facing shores will remain small with mainly background energy.

HFO Watches/Warnings/Advisories

High Surf Advisory until 6 AM HST Monday for Niihau, Kauai Leeward, Waianae Coast, Oahu North Shore, Maui Windward West, Kauai North, Molokai Windward, Molokai North, Molokai West, Maui Central Valley North, Windward Haleakala.
Flood Watch from late tonight through Tuesday afternoon for Maui Windward West, Maui Leeward West, Haleakala Summit, Kona, Kohala, Big Island Interior, Maui Central Valley North, Maui Central Valley South, Windward Haleakala, Kipahulu, South Maui/Upcountry, South Haleakala, Big Island South, Big Island Southeast, Big Island East, Big Island North.

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Data Courtesy of NOAA.gov